Heat sink



FIG. 1 is a top view of a heat sink showing my new design;

FIG. 2 is a front view thereof;

FIG. 3 is a side view thereof;

FIG. 4 is a front perspective view thereof; and,

FIG. 5 is a back perspective view thereof.

The broken-line disclosure forms no part of the claimed design. 

The ornamental design for a heat sink, as shown. 